Field of the Invention
The invention lies in the field of semiconductor manufacture. Specifically, the invention relates to a method of producing a light-emitting diode.
The term material layers is intended here to mean both layers of a single material and layer sequences or layer structures of different materials.
The production of products from semiconductors, for example electronic and optoelectronic components, typically requires a plurality of process steps, including the processes needed for growing semiconductor crystals and semiconductor layers, and for selective local removal and structuring of the layers. Many components consist in part of layer sequences of non-identical semiconductor materials, which are epitaxially grown in monocrystalline form on a substrate.
As the process steps for structuring semiconductor layers or for separating two semiconductor layers from one another, etching processes are customarily used which erode the semiconductor layers starting from the semiconductor surface. Such processes often take place very slowly and require corrosive chemicals. Further, not every known semiconductor material system has an etching process which allows corresponding layers to be structured with tolerable outlay.
In particular, the semiconductor materials indium nitride, gallium nitride and aluminum nitride (InN, GaN and AlN) and mixed crystals or alloys thereof, which will be referred to together in the text below as “group III nitrides”, are very difficult to etch chemically. No reliable wet chemical etching process is currently available for this material system. It is therefore necessary to use the technically very elaborate process of reactive ion etching (dry etching). However, this process allows only relatively low etching rates and requires poisonous and toxic gases (for example boron trichloride). Because etching processes act on the surface, it is usually necessary to control the rate and duration of the etching accurately in order to reach the desired depth.
Further, for some semiconductor materials, for example and in particular for group III nitrides, bulk crystals of them or of lattice-matched semiconductor materials cannot be produced, or can be produced only with great technical outlay. Substrates for growing such semiconductor layers are therefore only of very limited availability. For this reason it is common practice, in order to grow these semiconductor layers, as a replacement to use substrates of other materials which have properties unsatisfactory for subsequent process steps or for the operation of the component. For the growth of group III nitride layers, these are, for example, sapphire or silicon carbide substrates.
These “replacement” substrates entail problems such as unsuitable atomic lattice spacings and different coefficients of thermal expansion, which have detrimental effects on the material quality of the semiconductor layers grown on them. Further, some process steps such as the known cleavage of semiconductor layers in order to produce resonator mirrors of laser diodes in GaAs, are difficult or even impossible with these substrates.
In order to overcome these problems, various processes alternative to etching have to date been proposed for separating semiconductor layers or other layers from one another or from a problematic substrate.
In E. Yablonovitch et al., Appl. Phys. Lett. 51, 2222 (1987), U.S. Pat. No. 4,846,931, Thomas J. Gmitter and E. Yablonovitch, Jul. 11, 1989, it has been proposed to implement AlAs sacrificial layers in the GaAs/AlAs material system during the production process, which can then be dissolved using wet chemical means. This makes it possible to separate layers or structures from the substrate. However, because of the low lateral etching rate, this process is very time-consuming. For group III nitrides, furthermore, there exists no wet chemical etchant.
U.S. Pat. No. 4,448,636 describes a process for removing metal films from a substrate. There, the metal film is heated by light. An organic sacrificial layer between the substrate and the metal film is vaporized by the heat delivered and allows the metal layer to be removed. These organic intermediate layers cannot be employed, in particular, in the epitaxial growth of group III nitrides.
A comparable process has been described for removing silicon dioxide layers from gallium arsenide in Y.-F. Lu, Y. Aoyagi, Jpn. J. Appl. Phys. 34, L1669 (1995). There, as well, an organic intermediate layer is heated by light absorption and the SiO2 layer is lifted off.
Y.-F. Lu et al., Jpn. J. Appl. Phys. 33, L324 (1994) further discloses the separation of SiO2 strips from a GaAs layer using an excimer laser.
German patent DE 35 08 469 C2 describes a process for structuring layer sequences applied to a transparent substrate, in which the layers to be structured are exposed to laser radiation locally through a transparent substrate, and this laser radiation is absorbed in the layer to be structured.
Further, so-called laser ablation has been applied to many material systems in order to remove material. In this process, however, the surface is always destructively eroded and separation in two parts that can be used further is not possible.
Specifically for group III nitrides, Leonard and Bedair, Appl. Phys. Lett. 68, 794 (1996) describe the etching of GaN with a laser pulse under HCl gas and attribute it to a photochemical reaction involving hydrochloric acid.
Morimoto, J. Electrochem. Soc. 121, 1383 (1974) and Groh et al., physica status solidi (a) 26, 353 (1974) describe the thermally activated decomposition of GaN.
In Kelly et al., Appl. Phys. Lett. 69 (12), Sept. 16, 1996, p .1749-51 it is shown that group III nitrides can be laser-induced to undergo thermally activated composition. However, that process likewise involves a process that acts on the surface of the semiconductor layer and, in particular, leads to the destruction of the surface.